Present day methodology of manufacturing shaped articles, by pressing in a mould a plastics composition which includes a fibrous reinforcing material is beset with a number of not-readily solved problems. These problems are particularly apparent when the article being produced is so shaped that the mould tool must be provided with undercut portions, and when the reinforcement is critical, i.e. when both the distribution and orientation of the fibres is a deciding factor in the desired quality of the final article. To enable the moulded article to be removed from a die having undercut portions, the die must comprise a plurality of parts which are capable of moving relatively to one another. In order to obtain a reinforcement in which the fibres are correctly aligned, it is necessary to manually place the reinforcing material directly into the channel or recess of the die.
When using present day moulds, shaped articles are normally produced in accordance with the aforegoing, with the aid of a pre-impregnated reinforcement (wet reinforcement), by which is meant a reinforcement which has previously been drenched with plastics composition in an amount sufficient to meet demands in respect of the shaped article intended, this reinforcement being placed in a recessed or channelled die and there compressed by means of an upper mould tool, whereupon excess plastics material is pressed out of the mould. The tightness of the mould is not critical in this respect, thereby enabling the use of so-called compression moulds, i.e. moulds in which the die comprises a plurality of parts capable of moving relative to one another, these parts being moved into their final, operative positions under the influence of the overlying mould tool. When an excessive amount of plastics material is used, the fact that some of this excess will be squeezed out through the junctions or flash surfaces between the various mould-tool parts has no decisive effect.
The use of a wet reinforcement in the aforedescribed manner is, inter alia, encumbered with two serious disadvantages, however. Firstly, the working environment is extremely poor, since the wet reinforcement must be handled manually, thereby exposing the workman to harmful gases eminating from the plastics composition. It would be very difficult and expensive to construct a work place for carrying out this method, in which all the requirements for a safe, non-toxic environment were satisfied. Secondly, the output rate of such moulds is very low, since the press must be shut down for relatively long periods of time, in order to enable the reinforcement to be placed in the die. Furthermore, difficulties can be experienced in positioning the reinforcing material correctly in the die, in precisely the manner intended, since this must be done directly in the mould tool. This is particularly true when manufacturing products whose shape is such as to render it impossible to pre-shape the reinforcements into simple, readily handled units.
Against this background, it would be desirable to work with a dry reinforcement, i.e. to position the reinforcement in the die in a dry condition, and not to introduce the plastics composition into the mould cavity until the mould has been closed by the upper mould tool. Among other things, this would mean that the plastics composition could be constantly held in closed spaces. The use of dry reinforcement, however, requires the mould cavity to be sealed, practically hermetically, since otherwise it is not possible to obtain the required pressure in the plastics composition charged to the mould cavity. It is particularly difficult to achieve the requisite tightness in a die which includes a plurality of parts capable of moving relative to one another, since dry reinforcing material is relatively spiky, i.e. fibres constantly protrude outwardly from the surfaces thereof, these fibres becoming firmly trapped between the tool parts or flash surfaces, thereby to impair the seal therebetween. This is difficult to avoid, since when uncompressed, the reinforcement material can have a volume which is at least equal to, and often greater than the volumetric capacity of the mould cavity, particularly when the reinforcement includes filler bodies, meaning that the reinforcement must be pressed down into the channel or recess in the die. As a result hereof, the reinforcement will lie relatively hard against the defining surfaces of the channel or recess, and can readily be pressed into any joints present between the mould-tool parts.